The process of electrostatographic printing includes the step of charging an imaging member to a substantially uniform potential to sensitize the surface thereof. A latent image is generated on the charged portion of the surface of the imaging member by, in electrophotographic printing, exposure to a light image from, for example, an original document being reproduced, a scanning laser beam, an LED source, etc. or, in electron beam imaging and ion deposition printing, deposition of charges on the imaging medium. The recorded latent image is then developed by bringing a developer material into contact therewith. This forms a toner powder image on the imaging member that is subsequently transferred to a substrate, such as paper. Finally, the toner powder image is permanently affixed to the substrate in image configuration, for example by heating and/or pressing the toner powder image.
It is known that maintaining the state of the material in the developer housing within an optimum range improves developability and transfer efficiency. To accomplish this, many printing systems use a variety of processes to maintain the state of the developer materials within the optimum range by monitoring and controlling one or more characteristics of the materials including, for example, temperature, humidity, charge, toner concentration (ratio of toner to carrier) and toner charge distribution.
However, even if the developer materials are maintained in an optimal state it has been observed that when running low area coverage jobs the developability and/or transfer efficiency can fall off due to changes in the materials state in the developer housing. This fall off in developability and/or transfer efficiency produces weak, mottled and/or streaky images and can cause the process controls to use all of the printer's operating space in trying to correct the problems. With existing printing devices, when running low area coverage jobs and a reduced image quality suspected to result from a fall off in developability or transfer efficiency is observed, it is known to address the problem by either changing the materials within the developer housing(s) or by running a large number of prints (e.g., 1-2 thousand) of a high area coverage document to remove “bad” toner from the developer housings.
U.S. Pat. No. 4,614,165, assigned to the assignee hereof, describes what has come to be known as a “trickle” development system. In brief, a trickle development system provides a replaceable cartridge of developer (“developer” in this context comprising toner and carrier) that trickles its contents at a predetermined rate into a developer housing. The housing is the repository of the developer that is directly available for development of an electrostatic latent image. The developer in the housing has a desired “TC” for best performance in image quality, transfer efficiency, and other possible measures. (“TC,” as used herein, is some measurable and/or controllable value derived at least in part from the relative amounts, by weight or otherwise, of toner and carrier in the developer; the TC can be a simple ratio or percentage, or can be a more sophisticated number with other values, such as charge, associated therewith.) The developer in the cartridge (the cartridge being sometimes called a “replenisher”) has a TC significantly different from that of the developer in the housing, but is designed to maintain the desired TC in the housing as the cartridge gradually introduces its developer into the housing.
Over the years the basic trickle concept has been refined. U.S. Pat. No. 6,248,496 teaches a practical application of the concept. U.S. Pat. No. 6,466,749 shows a feedback-control system in which pure toner and pure carrier are available through separate controllable “taps” to maintain a desired TC within a developer housing.
Another strategy for maintaining a desired TC is to provide occasional “purges” of residual developer, such as by causing the printer to output images having high toner density. An example of such a technique is shown in U.S. Published Patent Application 2004/0170442-A1.
Also of interest to the present disclosure are: U.S. Pat. No. 5,585,899, which shows multiple replenisher cartridges associated with a single development unit; and U.S. Pat. No. 6,798,997, which shows network-based automated reordering of toner cartridges as needed by a machine.